The invention relates to a supply method and apparatus for operating a fuel cell stack.
Fuel cell stacks are considered to be a promising energy source, with a plurality of uses. The development of the fuel cells is pursued intensely in the automotive industry as an environmentally friendly energy source for vehicles.
Fuel cell stacks usually comprise a plurality of fuel cells, which generate electric energy from a fuel (often hydrogen) and an oxidant (often ambient air) via an electrochemical process. For this purpose, each of the individual fuel cells has an anode and a cathode region, which are separated from one another by a membrane. Fuel is supplied to the anode region, and oxidant is provided to the cathode region. The electric current is generated by catalytic means with support of transport processes by the interposed membrane.
Even though the basic principle of the fuel cell has, long been known, such intensive developments always lead to new issues with the practical realization of the fuel cell.
International patent document WO 02/23657 A2 focuses for example on the increase of the durability of fuel cell arrangements, by providing a protective housing around the fuel cells. The interior enclosed by the protective housing is switched in a closed circulation circuit to ensure even operating conditions for the fuel cells.
German patent document DE 100 56 536 A1 discloses a fuel cell arrangement wherein, instead of being recirculated, residual anode gases are continually mixed with supply air in a housing enclosing the fuel cell stack, and are then conducted through the cathode section of the fuel cell stack.
German patent document DE 10 2004 055.158 A1, which may show the closest state of the art, discloses a fuel cell system and a method of operating such a fuel cell system, wherein an exhaust gas line of a fuel cell stack on the anode side leads to a suction side of a compressor, into a supply line of the oxidant. During a purge (that is, a temporary purging discharge of the anode gases), the purge gases are mixed with the main oxidant flow, compressed by the compressor, and conducted to the cathode input of the fuel cell stack.
One object of the present invention is to provide a fuel supply system, and a method for its operation, which provide an improved operation of a fuel cell stack.
This and other objects and advantages are achieved by the fuel supply system according to the invention, which is suitable to supply gas to at least one fuel cell stack. The fuel cell stack has a plurality of fuel cells and is preferably formed for operation in a vehicle to provide drive energy for the vehicle. The fuel cell stack may be based on PEM (Proton-Exchange Membrane) technology.
The supply system according to the invention has a cathode gas supply line, which is configured to conduct an oxidant (in particular, ambient air) to a cathode input of the fuel cell stack. An anode gas supply line is configured to conduct fuel (in particular hydrogen) to an anode input of the fuel cell stack. The anode input or the cathode input are connected in a flow-technological manner with the cathode or anode section of the fuel cells.
The partially used anode gas exiting from the anode output is passed back to the anode input, via an anode recirculation line. The use of the anode gas recirculation line takes into consideration the circumstance that the supplied fuel (or hydrogen) is not completely converted electrochemically in the fuel cell stack. The remaining fuel or hydrogen is thus passed back by the anode gas recirculation line, refreshed with fresh (that is, unused) fuel or hydrogen, and passed back to the fuel cell stack via the anode input.
The supply system according to the invention may also include a purge line, which is arranged and/or formed to discharge partially used anode gas from the anode gas recirculation line into the cathode gas recirculation line. (Herein, the discharged partially used anode gas is called “purge gas”.) The anode gas is discharged as purge gas as soon as contaminants in the recirculated anode gas have exceeded predetermined limit values and have a disadvantageous effect on the operation of the fuel cell stack.
According to the invention, the supply system comprises a conditioning arrangement upstream of the cathode input. The conditioning arrangement is formed to compress or compact, and/or to accelerate and/or heat a partial flow of the total oxidant supplied to the fuel cell stack, and comprises a mixing station, where the partial flow is combined with the purge gas to form a mixed flow. The partial flow of the oxidant may be heated only when it is mixed with the purge gas. The conditioning arrangement conducts the mixed flow into the cathode gas supply line, thereby implementing three functions, namely the supply of energy to the partial flow or the mixed flow, the mixing of purge gas and partial flow, and the introduction of the mixed flow into the cathode gas supply line.
Applicants have noted that a pre-treatment of the partial flow of the oxidant prior to the mixing with the purge gas, and/or heating of the mixed flow prior to the introduction into the cathode gas supply line permits a very exact control over the condition of the total flow mixture supplied to the fuel cell stack. The added catalytic conversion of the residual hydrogen contained in the purge gas, and thus in the total flow mixture, can be carried out trouble-free due to the improved condition control. In particular in the circumstance that only a partial flow (but not all) of the oxidant may be pretreated as mixed flow, it improves the control over the condition of the resulting total flow mixture.
A preferred embodiment of the supply system according to the invention further comprises a purge valve, via which the purge line can be connected. The purge valve is particularly accessed by a control device, which initiates a purge depending, for example, on i) the energy taken from the fuel cell stack, ii) the time, or iii) measured contaminants in the anode gas. It is again made clear in this embodiment that the purge line is passed through only intermittently, namely during a purge.
In a practical implementation of the supply system according to the invention, the cathode gas supply line comprises a first pump, and the discharge of the conditioning arrangement leads upstream of the first pump into the cathode gas supply line. The first pump thus conveys the mixed flow and the part of the oxidant flow which is not conducted through the conditioning arrangement. The first pump may be of arbitrary construction, in particular as a displacement pump, fluid flow engine, or flow pump.
In a further embodiment of this practical implementation, the supply line for the partial flow of the oxidant branches into the conditioning arrangement in front of the first pump and/or in front of the discharge of the conditioning arrangement. In this embodiment, it is emphasized again that the supply to and discharge from the conditioning arrangement lead into the cathode supply line upstream of the first pump.
In a preferred embodiment of the supply system, the conditioning arrangement comprises a second pump for accelerating, or compressing and/or compacting the particularly unmixed partial flow of the oxidant. The latter pump may be controlled or regulated by a control device, so that the energy conducted to the partial flow by the second pump can be adjusted.
In an advantageous further embodiment, the supply system has a housing which is formed especially for the gas-tight reception of the fuel cell stack(s). The housing has flow-through regions, which are thermally coupled to the fuel cell stack(s). In particular, the thermal coupling comprises a heat exchanger, and the fuel cell stack(s) are used for heating gases flowing through the flow-through regions during operation. At least one part of the flow-through regions, preferably part of the conditioning arrangement or associated therewith, is formed and/or arranged for conducting the mixed flow. The mixed flow is heated by the thermal energy released by the fuel cells during the operation in this arrangement.
In another alternative of the embodiment, the flow-through regions or at least a part thereof are formed for heating the unmixed partial flow. The partial flow is thus compacted and heated before it is mixed with the purge gas at the mixing station.
In a third possible alternative of the embodiment, the mixing station is arranged within the housing (in particular in the flow-through regions), so that the supplied unmixed partial flow, the supplied unmixed purge gas, and the discharged mixed flow are heated.
All three possible alternatives have the advantage that additional energy can be conducted to the gases for the total energy balance in an economical manner.
The invention also relates to a method for operating a supply system such as described above in a vehicle, wherein an anode purge is carried out by connecting the purge line after reaching a critical contamination of the recirculated anode gas, so that the contaminated anode gases are conducted from the anode gas recirculation line into the conditioning arrangement.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.